The mechanism(s) by which microtubule plus-end tracking proteins are targeted is unknown. In the filamentous fungus Aspergillus nidulans, both cytoplasmic dynein and NUDF, the homolog of the LIS1 protein, localize to microtubule plus ends as comet-like structures. Herein, we show that NUDM, the p150 subunit of dynactin, also forms dynamic comet-like structures at microtubule plus ends. By examining proteins tagged with green fluorescent protein in different loss-offunction mutants, we demonstrate that dynactin and cytoplasmic dynein require each other for microtubule plus-end accumulation, and the presence of cytoplasmic dynein is also important for NUDF's plus-end accumulation. Interestingly, deletion of NUDF increases the overall accumulation of dynein and dynactin at plus ends, suggesting that NUDF may facilitate minus-enddirected dynein movement. Finally, we demonstrate that a conventional kinesin, KINA, is required for the microtubule plus-end accumulation of cytoplasmic dynein and dynactin, but not of NUDF.
INTRODUCTIONIn eukaryotic cells, the microtubule cytoskeleton is essential for cell cycle progression, the establishment of cell polarity, and cell migration. In most interphase cells, microtubules are polarized in such a way that the minus ends are located at the microtubule organizing center near the nucleus while the plus ends extend to the periphery. Microtubule plus ends are very dynamic, constantly exploring the cytoplasmic space with alternate growing and shrinking phases (reviewed by Desai and Mitchison, 1997;Gundersen, 2002). Kinesin superfamily members and cytoplasmic dynein together play important roles in intracellular trafficking of various proteins, vesicles, and organelles. Because conventional kinesin is a plus-end-directed motor and dynein is a minus-end-directed motor, it is thought that conventional kinesin is responsible for moving materials from the cell body toward cell periphery, whereas cytoplasmic dynein moves materials from the cell periphery back to the cell body (reviewed by Goldstein and Yang, 2000).We have shown that cytoplasmic dynein is required for the proper distribution of nuclei within the hyphae of the filamentous fungus Aspergillus nidulans (reviewed by Morris et al., 1998b). Cytoplasmic dynein is a complex that consists of heavy chains (HCs), intermediate chains (ICs), light intermediate chains, and light chains (reviewed by King, 2000;Tynan et al., 2000). The HC contains the motor domain (reviewed by Asai and Koonce, 2001), and the other chains may target the motor to various cargoes (Steffen et al., 1997;Tai et al., 1999;Young et al., 2000). The function of cytoplasmic dynein requires dynactin (reviewed by Holleran et al., 1998;Ma et al., 1999;Roghi and Allan, 1999;King and Schroer, 2000;Kumar et al., 2000), a complex that contains multiple subunits, including the 150-kDa dynactin protein and the actin-related proteins Arp1 and Arp11 (reviewed by Holleran et al., 1998;Eckley et al., 1999). Our genetic analyses of nuclear distribution identified multiple nuclear d...